Ablation as used hereinafter refers to the dissipation of heat from an object generally by use of a material which is sacrificially eroded or ablated. Ablation is extremely important in the aerospace industry in that spacecraft, such as reuseable launch rockets and space shuttles, are subjected to extremely high temperatures as a result of heat generated by atmospheric friction on reentry into the earth's atmosphere. The heat which is generated, if not removed from the spacecraft, can destroy the entire spacecraft and/or the heat sensitive components of the spacecraft. In order to insure the survival of the spacecraft and the heat sensitive components on reentry, it has become standard practice in the aerospace industry to cover critical surface areas, such as the nose cone and leading edges of the spacecraft, with materials that ideally should erode or ablate slowly and smoothly in such a manner as to protect the spacecraft from the generated heat of reentry. In addition to the problems encountered as a result of the heat generated on reentry, problems concerning ablation are also of considerable interest to the aerospace industry in other areas, such as the protection of rocket nozzle structures from attrition by hot propellant gases and the insulation of rocket motor case structures from the heat of burning propellant. A more detailed description of ablation and the problems encountered as a result of ablation are discussed in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 1, pp. 11-21, which is incorporated by reference herein.
Numerous materials have heretofore been suggested for use as ablative materials but none have proven to be entirely suitable. The materials heretofore suggested have included organic, inorganic and composite materials. A class of materials which has been widely used and is generally considered to be the most suitable is the phenolic resins. The phenolic resins have certain properties which make them especially useful as ablative materials, such as forming a coke-like char and releasing low-molecular-weight gases on ablating which results in more effective dissipation of heat. Phenolic resins which have heretofore been used, however, did not by themselves have sufficient physical strength for use in many ablative applications and required reinforcement with substantial amounts of strong fibrous materials, such as asbestos, glass or nylon fibers. Other organic materials have also been employed to some extent as ablative materials such as the polyterephthalate fibers. Inorganic materials which were suggested have included graphite which has good ablative properties but has limited physical strength so that it cannot be used in many applications. Refractory ablative materials have also been suggested and likewise were found to be generally unsuitable for most applications because of the lack of resistance to thermal shock.
Another area of concern which is closely related to the problem of ablation is the provision of fabrics and the like which have decreased flammability. This is a particularly pressing problem in recent years because of the elimination of the use of asbestos in fabrics.
What whould be highly desirable would be a material which would have sufficient ablative properties so as to effectively dissipate heat in aerospace applications and which would not have the shortcomings, particularly low strength, of prior art materials. It would likewise be especially desirable if the material would exhibit low flammability so as to be usable in other applications such as protective clothing and the like.